Genetic differences exist among individual animals as well as among breeds which can be exploited by breeding techniques to achieve animals with desirable characteristics. For example, Chinese breeds are known for reaching puberty at an early age and for their large litter size, while American breeds are known for their greater growth rates and leanness. However, heritability for desired traits is often low, and standard breeding methods which select individuals based upon phenotypic variations do not take fully into account genetic variability or complex gene interactions which exist.
Restriction fragment length polymorphism (RFLP) analysis has been used by several groups to study pig DNA. Jung et al., Theor. Appl. Genet., 77:271–274 (1989), incorporated herein by reference, discloses the use of RFLP techniques to show genetic variability between two pig breeds. Polymorphism was demonstrated for swine leukocyte antigen (SLA) Class I genes in these breeds. Hoganson et al., Abstract for Annual Meeting of Midwestern Section of the American Society of Animal Science, Mar. 26–28, 1990, incorporated herein by reference, reports on the polymorphism of swine major histocompatibility complex (MHC) genes for Chinese pigs, also demonstrated by RFLP analysis. Jung et al., Theor. Appl. Genet., 77:271–274 (1989), incorporated herein by reference, reports on RFLP analysis of SLA Class I genes in certain boars. The authors state that the results suggest that there may be an association between swine SLA/MHC Class I genes and production and performance traits. They further state that the use of SLA Class I restriction fragments, as genetic markers, may have potential in the future for improving pig growth performance.
The ability to follow a specific favorable genetic allele involves a novel and lengthy process of the identification of a DNA molecular marker for a major effect gene. The marker may be linked to a single gene with a major effect or linked to a number of genes with additive effects. DNA markers have several advantages; segregation is easy to measure and is unambiguous, and DNA markers are co-dominant, i.e., heterozygous and homozygous animals can be distinctively identified. Once a marker system is established selection decisions could be made very easily, since DNA markers can be assayed any time after a tissue or blood sample can be collected from the individual infant animal, or even an embryo.
The use of genetic differences in receptor genes has become a valuable marker system for selection. For example, U.S. Pat. Nos. 5,550,024 and 5,374,526 issued to Rothschild et al. disclose a polymorphism in the pig estrogen receptor gene which is associated with larger litter size, the disclosure of which is incorporated herein by reference. U.S. Pat. No. 5,935,784 discloses polymorphic markers in the pig prolactin receptor gene which are associated with larger litter size and overall reproductive efficiency.
Litter size, of course has a direct economic impact for a breeder. Also important for meat producing animals is growth, appetite and fatness.
The quality of raw pig meat is influenced by a large number of genetic and non-genetic factors. The latter include farm, transport, slaughter and processing conditions. Meat scientists have performed a substantial amount of research on these factors, which has led to considerable quality improvement. Part of the research has also been dedicated to the genetic background of the animals, and several studies have revealed the importance of genetic factors. This has made the industry aware that selective breeding of animals and the use of gene technology can play an important role in enhancing pork quality.
Information at the DNA level can not only help to fix a specific major gene, but it can also assist the selection of quantitative traits for which we already select. Molecular information in addition to phenotypic data can increase the accuracy of selection and therefore the selection response. The size of the extra response in such a Marker Assisted Selection (MAS) program has been considered by many workers from a theoretical point of view. In general terms, MAS is more beneficial for traits with a low heritability and which are expensive to measure phenotypically. Although traits such as growth, appetite and fatness are not typically considered in this way, there are still significant advantages for the use of markers for these traits. For example, Meuwissen and Goddard considered the impact of MAS for different types of traits. The biggest impacts were for traits such as meat quality, where the trait is measured after slaughter and an additional response of up to 64% could be achieved with the incorporation of marker information. This figure was relatively small, 8%, for growth traits that can be measured on the live animal. However, once the association has been demonstrated this marker information can be used before the animals are tested or selected phenotypically (see below) and in this situation a response of up to 38% was predicted.
Indeed, the best approach to genetically improve economic traits is to find relevant DNA-markers directly in the population under selection. Phenotypic measurements can be performed continuously on some animals from the nucleus populations of breeding organizations. Since a full assessment of most of these traits can only be done after slaughter, the data must be collected on culled animals and cannot be obtained on potential breeding animals.
This phenotypic data is collected in order to enable the detection of relevant DNA markers, and to validate markers identified using experimental populations or to test candidate genes. Significant markers or genes can then be included directly in the selection process. An advantage of the molecular information is that we can obtain it at a very young age in the breeding animal, which means that animals can be preselected based on DNA markers before the growing performance test is completed. This is a great advantage for the overall testing and selection system.
It can be seen from the foregoing that a need exists for identification of markers which may be used to improve economically beneficial characteristics in animals by identifying and selecting animals with the improved characteristics at the genetic level.
An object of the present invention is to provide a genetic marker based on or within the Ghrelin gene which is indicative of favorable economic characteristics such as growth, appetite and fatness.
Another object of the invention is to provide an assay for determining the presence of this genetic marker.
A further object of the invention is to provide a method of evaluating animals that increases accuracy of selection and breeding methods for the desired traits.
Yet another object of the invention is to provide a PCR amplification test which will greatly expedite the determination of presence of the marker.
Additional objects and advantages of the invention will be set forth in part in the description that follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The objects and advantages of the invention will be attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.